1. Field of The Invention
The present invention relates generally to an evaporation control system for an internal combustion engine. More particularly, the invention relates to a fuel vapor purging control system including an air/fuel ratio compensating system which is operable to assure a desired air/fuel ratio of an air/fuel mixture entering combustion chambers of the engine whether or not fuel vapor purging control is carried out.
2. Description of The Prior Art
Japanese Patent First Publication No. 62-233466 discloses a fuel vapor purging control system for an internal combustion engine which stores in a canister fuel vapors subsequently generated from a fuel tank before they can escape to the atmosphere, and supplies the fuel vapors to an induction system of the engine according to an engine intake vacuum level. In this conventional purging control system, the fuel vapors in the canister are purged first at a relatively lower rate. An air/fuel sensor arranged in an exhaust system of the engine monitors the oxygen concentration remaining in exhaust gas to project an air/fuel ratio which is subjected to change caused by the purged fuel vapors. When a deviation of the air/fuel ratio from a desired value is less than a preselected threshold value, the purge flow rate is increased. Alternatively, when the deviation is greater than the threshold value, the relatively lower purge flow rate is maintained as is. This prevents the air/fuel ratio from varying, which may be caused by the purged fuel vapors introduced into the induction system of the engine.
The above prior art purging control system, however, rises a drawback in that it is impossible to determine a total amount of fuel supplied into combustion chambers of the engine as the amount of fuel contained in the purged fuel vapors cannot be determined before they go into the engine. Therefore, a variation in air/fuel ratio caused by the purged fuel vapors is not compensated precisely.